Semiconductor storage devices are used in integrated circuits for electronic applications, including radios, televisions, cell phones, and personal computing devices. Storage devices include charge-storing devices such as dynamic random access memories (DRAMs) and flash memories.
A more recent development in storage devices involves spin electronics, which combine semiconductor technology and magnetic materials. The spin polarization of electrons, rather than the charge of the electrons, is used to indicate the state of “1” or “0.” One type of semiconductor memory device is magnetoresistive random access memory (MRAM), which involves spin electronics that combines semiconductor technology and magnetic materials and devices. The spins of electrons, through their magnetic moments, rather than the charge of the electrons, are used to indicate bit values.
A typical MRAM cell may include a magnetic tunnel junction (MTJ) device, which generally includes a free layer, a pinned layer, and a tunnel layer interposed between the free layer and the pinned layer. The magnetization direction of the free layer can be reversed by applying a current through the tunnel layer, which causes the injected polarized electrons within the free layer to exert spin torques on the magnetization of the free layer. The pinned layer has a fixed magnetization direction. When current flows in the direction from the free layer to the pinned layer, electrons flow in a reverse direction, that is, from the pinned layer to the free layer. The electrons are polarized to the same magnetization direction of the pinned layer after passing the pinned layer, flowing through the tunnel layer, and then into and accumulating in the free layer. Eventually, the magnetization of the free layer is parallel to that of the pinned layer, and the MTJ device will be at a low resistance state. The electron injection caused by current is referred to as a major injection.
When current flowing from the pinned layer to the free layer is applied, electrons flow in the direction from the free layer to the pinned layer. The electrons having the same polarization as the magnetization direction of the pinned layer are able to flow through the tunnel layer and into the pinned layer. Conversely, electrons with a polarization differing from the magnetization of the pinned layer will be reflected (blocked) by the pinned layer, and will accumulate in the free layer. Eventually, magnetization of the free layer becomes anti-parallel to that of the pinned layer, and the MTJ device will be at a high-resistance state. The respective electron injection caused by current is referred to as a minor injection.